The present application relates to a signal decoding apparatus, a signal decoding method, a program, and an information record medium that accomplish for example high-speed, large-capacity mobile communications.
In recent years, as the number of accesses to the Internet through wireless communications has increased, needs of high-speed and large-capacity packet communications have drastically become large. When high-speed wireless data communications are made, since the signal bandwidths become large, communication quality tends to deteriorate due to frequency selective fading.
Against this problem, Orthogonal Frequency Division Multiplex (OFDM) is a communication system that has an excellent frequency fading resistance. IEEE 802.11a/g system, which is a widely used wireless LAN system, performs packet communications according to OFDM. However, wireless LAN is a system expected to communicate at hotspots in offices, which are in a semi-stationary state. Thus, in high-speed mobile environments such as cars, since changes of fading channels are not correctly followed up, communication characteristics deteriorate.
As one of countermeasures to enhance the resistance against a radio transmission environment in a high-speed mobile communication state, signals distorted through multi-path fading are compensated by estimating a radio fading channel. With Channel Estimation (CE) symbols inserted into transmitted signals and received signals, changes of fading channels are estimated. Based on the estimated results, signals are compensated. This countermeasure is described in Japanese Patent Laid-Open Publication No. 2002-271293.
In next-generation mobile wireless access systems, maximum transmission rates of several hundred Mbps to several Gbps are being accomplished. As a technology that realizes such high transmission rates along with high spectral efficiency, Space Division Multiplexing (SDM) is being spotlighted.
SDM is a transmission system of which a transmitter uses multiple antennas, transmits different signals from these antennas, and spatially multiplexes the signals. With this multiplexing, transmission rates proportional to the number of transmission antennas can be obtained without necessity of widening frequency bandwidths.
As a factor that affects transmission performance in SDM, there are signal detection algorithms used for receivers. These algorithms are mainly categorized as two types, one of which is spatial filtering based on Zero-Forcing (ZF) and Minimum Mean Square Error (MMSE) criteria and other of which is Maximum Likelihood Detection (MLD). The former has a merit of which the amount of calculation is small and a demerit of which the transmission capacity is low. In contrast, MLD has a merit of which the transmission capacity is excellent and a demerit of which the amount of calculation is huge.
For MLD, a technique that keeps as much transmission capacity as possible and reduces the amount of calculation has been proposed. The inventor of the present patent application has proposed a signal detection algorithm that separates SDM signals according to Multiple-QR-Decomposition (Multi-QRD).
As another important factor of SDM, there is a channel estimation for a receiver. The foregoing signal detection algorithms operate based on channel responses among transmission and reception antennas. Thus, the accuracies of channel estimations of the algorithms that estimate channel responses largely affect the operations thereof. With respect to channel estimations, many approaches have been made. In particular, many channel estimations using pilot symbols have been proposed.
When SDM is applied to a mobile wireless communication system, as a terminal travels, channels change due to high-speed fading, resulting in a new problem for channel estimations. When changes of channels are not able to be ignored against a time period of transmission signals, errors of channel estimations using only pilot symbols become large in the signal time period. As a result, there is a possibility of which transmission characteristics deteriorate. The foregoing related art estimates only a single radio fading channel.